Nucleation of misfit dislocations in strained-layer InGaAs on GaAs

Abstract

Austenitic 316L stainless steel alloys annealed at 550°C for 100 h present a few ferrite precipitates surrounded by a new interfacial phase, here called the I-phase, that develops at the level of the austenite/ferrite interface. The I-phase presents the typical patterns of an icosahedral quasicrystal with a primitive hypercubic lattice of parameter A = 0.63 nm. The marked orientation relationships between the I-phase and the ferrite precipitates strongly suggest that this phase results from a decomposition of the ferrite and not of the austenite. The I-phase is metastable and transforms eventually after annealing at 700°C to the stable crystalline σ-phase.     

Screw misfit dislocations perpendicular to one or two free surfaces

ABSTRACT

When a screw dislocation pierces one or two free surface(s), noticeable elastic relaxation takes place nearby these surfaces. To refine the theoretical interpretation of an electron microscopy image this relaxation should be included in a precise imaging model. In the present work, the relaxation field is evaluated for a periodic set of misfit screw dislocations normal to the free surface of a semi-infinite or plate-like heterogeneous bicrystal. The proposed approach uses an appropriate combination of known biperiodic elastic fields. For a homogeneous medium and when the period increases, the results around a misfit screw dislocation converge to those of Eshelby and Stroh (1951) who considered an isolated translation screw dislocation.     

On the determination of the nature of misfit dislocations in semiconductor strained-layer heterostructures

Abstract

The study of the mechanisms of formation of misfit dislocations in latticemismatched semiconductor interfaces requires a comprehensive characterization of these dislocations. In particular the determination of the sense of their Burgers vectors by transmission electron microscopy makes it possible to show that they contribute to removing part of the mismatched-induced interfacial strain. Furthermore the nature (αorβ) of the dislocations can then be determined. The different methods of determination of the sense of Burgers vectors are reviewed and then applied to InGaAs/GaAs superlattices.     

Shape deformation by moving a glissile interface with one set of misfit dislocations

Transformation and misfit dislocations are used to describe the motion of glissile interfaces with one set of misfit dislocations in the framework of the Frank?Bilby equation. The sweep of these glissile interfaces brings about an invariant-plane-strain type shape deformation. Our approach explains the glissile motion of martensitic interfaces and small-angle symmetrical tilt grain boundaries. It is consistent with the phenomenological theory of martensite crystallography but more flexible.     

A branch-and-bound algorithm for fitting anti-robinson structures to symmetric dissimilarity matrices

The seriation of proximity matrices is an important problem in combinatorial data analysis and can be conducted using a variety of objective criteria. Some of the most popular criteria for evaluating an ordering of objects are based on (anti-) Robinson forms, which reflect the pattern of elements within each row and/or column of the reordered matrix when moving away from the main diagonal. This paper presents a branch-and-bound algorithm that can be used to seriate a symmetric dissimilarity matrix by identifying a reordering of rows and columns of the matrix optimizing an anti-Robinson criterion. Computational results are provided for several proximity matrices from the literature using four different anti-Robinson criteria. The results suggest that with respect to computational efficiency, the branch-and-bound algorithm is generally competitive with dynamic programming. Further, because it requires much less storage than dynamic programming, the branch-and-bound algorithm can provide guaranteed optimal solutions for matrices that are too large for dynamic programming implementations.     

Formation and structure of misfit dislocations at the La2Zr2O7-Y2O3 -stabilized ZrO2 (001) reaction front during vapour-solid reaction

La₂ Zr₂ O₇ (LZO)-based pyrochlore islands were grown on a Y₂ O₃ -stabilized ZrO₂ (YSZ) (001) single-crystal surface by the reaction between La₂O₃ vapour and the crystal. A network of interfacial edge dislocations with line directions [100] and [010] and Burgers vectors ( a_s/2)[101] and ( a_s/2)[011] respectively ( a_s being the lattice parameter of the YSZ) was observed at the moving LZO-YSZ reaction front. The interface-parallel component of the Burgers vectors accommodates the LZO-YSZ lattice mismatch of +5.0%, while the perpendicular component causes a slight tilt of the LZO lattice with respect to the YSZ lattice. The dislocation half-loops nucleate and glide on inclined {101} planes at the edges of the four corners of the growing islands.     

Cathodoluminescence-mode imaging of dislocations in zinc oxide

Abstract

The cathodoluminescence mode in the scanning electron microscope has been used to image bands of dislocations in zinc oxide. It has been shown that dislocations lying on pyramidal, {1012}, planes are generated beneath indentations on {0001} surfaces. Basal plane dislocations lying parallel to the surface are also observed. Indentations on {1100} prismatic surfaces result in slip on {0001} and {0110} planes.     

A dynamic multilevel model of demographic diversity and misfit effects

In this study, the authors proposed and evaluated the linkages of a dynamic multilevel model of demographic diversity and misfit effects in a large sample of quick-service restaurants. Using a cross-level approach, the authors studied an employee's demographic misfit in relation to coworkers' demographics as a predictor of turnover risk over time. At the business-unit level, they studied changes in restaurant demographic diversity in relation to changes in profitability over time and unit turnover rates in relation to profitability. The authors also studied the impact of the match between the racial compositions of the restaurants and their communities on profitability. The results supported linkages between demographic misfit and turnover and partially supported a negative association between racial diversity and changes in profitability.     

Transmission electron microscopy of dislocations in SrTiO3

Dislocations have been introduced in SrTiO₃ by Vickers indentation at room temperature and analysed by transmission electron microscopy. The slip systems in SrTiO₃ were identified as ?110?-{110}. ?110? dislocations are dissociated into two partial dislocations. The stacking-fault energy γSF was determined to be 136 ± 15 mJm^-2.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Deformation-induced dislocations in 15R-SiC grown by sublimation

Single-crystal 15R-SiC boules have been successfully grown by sublimation. The Vickers hardness of a Si-terminated (0001) face has been measured in the temperature range 25-1300C. As expected, the hardness decreases with increasing temperature from about 30GPa at room temperature to about 10GPa at 1300C. The fracture toughness is estimated to be about 1.0MPam 1/2 at room temperature. Transmission electron microscopy investigation of the dislocations introduced by indentation at 900 and 1300C shows that they are activated predominantly on the basal plane. Most of them consist of a single leading partial without the corresponding trailing partial.     

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.     

On the asymmetrical dissociation of Lomer dislocations

Abstract

Using anisotropic elasticity theory, Korner, Schmid and Prinz have shown that Lomer-Cottrell dislocations in the f.c.c. structure have asymmetrical equilibrium positions. This is also true for the isotropic theory, but in addition it can also be demonstrated in a very simple way that the ratio of the dissociation widths on the two {111} planes is material independent. This result seems to be general even for the anisotropic case. However, careful observations of Lomer dislocations in copper by weak-beam transmission electron microscopy indicate clearly that they are not dissociated.     

Stair-rod dislocations in perovskite films on LaAlO3 substrates

Two types of stair-rod dislocation formed at junctions of {111} stacking faults in SrTiO₃-SrRuO₃ two-layer films on LaAlO₃ substrates have been studied by high-resolution transmission electron microscopy. The first type, formed by the interaction of two stacking faults situated at the flat SrRuO₃-LaAlO₃ interface without any step, has a Burgers vector (a/3)<110>. The other type, formed by the interaction of two stacking faults, of which one starts from the SrRuO₃-LaAlO₃interface area involving an interface step, has a Burgers vector (a/3)<120>. The formation mechanism of the stair-rod dislocations and the effect of the step are discussed.     

Accumulation of geometrically necessary dislocations near grain boundaries in deformed copper

Cross-correlation-based analysis of electron backscatter diffraction patterns has been used to map the distribution of geometrically necessary dislocation (GND) density in deformed polycrystalline copper. Patterning of the dislocations into high-density cell walls and low-density cell interiors was readily observed at the micron scale. Patterning at the longer length scale of the grain size was also evident with high-density regions (GND hot spots) tending to be in clusters, often found close to some but not all grain boundaries and triple junctions.     

Real-space first-principles electronic structure of edge dislocations: NiAl

The electronic structure of the 100 {010} edge dislocation in NiAl has been calculated using the real-space tight-binding linear muffin-tin orbital recursion method with a self-consistent treatment of electron density redistribution effects in the dislocation core. We demonstrate that quasilocalized states may exist in this metallic system as a result of specific lattice distortions in the dislocation core with 'broken bonds'.     

HREM examination of [101] screw dislocations in Ni3Al

Abstract

The dissociation of [101] screw dislocations in Ni³Al has been examined using high-resolution electron microscopy. [101] superdislocations are found to be dissociated into (a/2)[101] superpartial dislocations on the (010) cube cross-slip plane. These superpartials in turn dissociate into complex stacking faults on the (111) or (111) which are bounded by Shockley partials in agreement with theoretical predictions. The degree of antiphase boundary spreading on (010) was found to increase with deformation temperature while the superpartial core dissociations remain unchanged.